The present invention relates to a pipe joint for connecting pipes and, more particularly, to electrically insulated pipe joints usable for connecting pipes to be laid underground with cathodic protection.
In conducting protection of, e.g., a buried natural gas pipeline against corrosion, it is necessary to electrically insulate the underground piping by providing an isolating pipe joint with each ground riser portion of the piping. Up to now, flange-type pipe joints and/or monolithic-type isolating joints have been predominantly used for the above purpose. A typical flange-type pipe joint is shown in FIG. 1, which includes weld neck flanges 1a, 1b, insulating sleeve 2, insulating washer 3, steel washer 4, insulating gasket ring for raised face 5, steel stud bolt 6, nut 7. However, the pipe joint of this type is so large and heavy that it cannot be easily handled and requires complicated assembling works for accurately centering bolt holes bored in weld neck flanges 1a, 1b and evenly fastening a number of bolts 6 and nuts 7. In particular, the flange-type pipe joints used for high-pressure pipeline may often be troubled with leakage of pressurized line content and in many cases cannot stop the leakage by tightening their fixing bolts.
An example of the monolithic-type isolating pipe joint is shown in section in FIG. 2, which includes short pipes 11a, 11b, steel-made parts 12, 13, 14, seal gaskets 15, isolating rings 16, 17, an insulating filler 18 and welds W.sub.1, W.sub.2, W.sub.3.
As shown in FIG. 2, the monolithic isolating pipe joint isolates left and right pipes (short pipes 11a and 11b) from each other by isolating rings 16 and 17, thus achieving considerable savings in size as compared with the flange-type pipe joint. However, the monolithic isolating joint takes many steps to manufacture. Namely, the steel parts 13 and 14 are welded with the welds W.sub.1 and W.sub.2 to the short pipes 11a and 11b respectively. The isolating rings 16 and 17 are mounted with the insulating filler 18 on the welded steel-made parts, then the short pipes 11a and 11b are butt-welded with the weld W.sub.3 while applying a specified pressure to the surface of the seal gasket 15. Thus, the monolithic isolating joint is so complex in design that it takes many steps to manufacture and requires several times higher cost than the flange-type insulating pipe joint.
On the other hand, there has been introduced a clamp-type pipe joint "GRAYLOC" (registered trademark) being a circumferentially self-clamping-type pipe joint to improve the conventional high-pressure pipe joints being large size, heavy and difficult to fasten and disassemble and very expensive to manufacture.
FIGS. 3A and 3B illustrate the clamp-type pipe joint GRAYLOC before and after fastening its clamp, which includes hubs 21a, 21b, a seal ring 22, a seal-ring rib 22R, a seal-ring contact surface 22L, a clamp 23 and a clamp contact surface S.
This pipe joint does not require aligning bolt-holes while it is needed for flange-type pipe joints. The GRAYLOC pipe joint is of self-clamping-type that has been well proven in practice to work with high reliability under high pressure of fluid. However, the GRAYLOC pipe joint has not been designed to assure sufficient electrical insulation between the pipes.